The present invention relates to a process for agglomerating a polymer powder, more particularly to an agglomeration process of a polymer powder which is prepared in a reversed phase suspension polymerization, reversed phase emulsion polymerization or polymerization in static state, and especially to an agglomeration process of a high water-absorbent resin powder.
Acrylic high water-absorbent resin powders have hitherto been prepared in a manner wherein an acrylic high water-absorbent polymer is prepared in reversed phase suspension polymerization, reversed phase emulsion polymerization or polymerization in static state, then the obtained polymer is dried as it is, or further, the dried one is pulverized. Also, resin powders other than the acrylic resin powders are prepared in suspension polymerization or emulsion polymerization in addition to the above-mentioned polymerization methods.
The resin powders prepared by the above-mentioned methods, however, have usually much fine powder which can pass through a 100 mesh standard sieve. Such resin powders have the following defects:
(1) Dusts are easily generated, the working surrounding becomes easily bad and final products easily lose in weight.
(2) The miscibility and dispersibility of the resin powder with another material are bad.
(3) It is easy to make undissolved lumps of the resin powder when contacting with liquid.
(4) Bridging and flushing are easily caused in a hopper due to poor fluidity of the resin powder.
For solving the above-mentioned defects, it has been proposed to remove the fine powder from the obtained powder or to agglomerate the powder, using a binder.
The former method is economically disadvantageous. The latter method is, when using an organic solvent binder, in danger of catching fire in a drying step of the obtained granules, and if insufficiently drying the granules, they are problematic in safety to human bodies due to the remaining organic solvent therein. When using, as the binder, a conventional aqueous liquid (for instance, water alone, an aqueous mixture of water and an organic solvent miscible with water or an aqueous solution wherein a water-soluble high molecular weight compound such as polyacrylic acid or polyvinyl alcohol is dissolved in water or the aqueous mixture as mentioned above), though there is no problem caused in the case that the organic solvent binder is used, it is difficult that the aqueous liquid is uniformly diffused, in other words, the aqueous liquid is unevenly distributed in the powder, and big lumps of the powder, having high density (hereinafter referred to as "block") are produced in the obtained granules. Thus, it is difficult to obtain uniform granules.
As an agglomeration process of a high water-absorbent resin powder which improves the above-mentioned defects, there are processes described in Japanese Unexamined Patent Publication No. 61-97333 and No. 61-101536, wherein a mixture of a high water-absorbent resin powder and an inorganic powder is stirred in a specific apparatus, and to the resulting mixture is added the aqueous liquid. According "to these processes, it is required to supply the aqueous liquid in the state of a fine droplet to the high water-absorbent resin powder, for uniformly mixing the resin powder with the aqueous liquid. When using the water-soluble high molecular weight compound as the binder, however, for supplying its aqueous solution in the state of a fine droplet to the resin powder, it is necessary to make the amount of the binder small or to dissolve the binder in a large amount of water because of its high viscosity. Thus, the obtained granules are poor in strength or it is required to expend much time and much energy in drying them. On the other hand, if the aqueous liquid is not supplied in the state of a fine droplet to the resin powder stirred at a high speed by using the specific apparatus, the uniform granules are not obtained and the blocks are generated partially.
Since the blocks impair the physical properties of the final product, it has been desired to develop a binder having an excellent diffusibility to resin powder.
It is an object of the present invention is to provide a process for agglomerating various kinds of resin powders having a fine powder which is difficult to handle, including high water-absorbent resin powders, by which granules having a particle size suitable for practical use and a narrow particle size distribution can be obtained.
This and other objects of the present invention will become apparent from the description hereinafter.